New, simple and inexpensive method of creating

Dr. Ing. Vadim Verlotski Thermico GmbH & Co.KG

New, simple and inexpensive method of creating highly flowable spray powders for HVOF and HV-APS from the finest metal and cermet powders (D50 < 5 microns): an industrially acceptable alternative to suspension spraying Introduction It is the nature of thermal spraying that the powder particles can only form a good layer if they get enough thermal and kinetic energy from the flame. It is understandable that the hotter and faster the flame is, the more energy can absorb the spray particles. However, that is not all. No less important is the particle size. The same flame is better "exploited" by finer particles than by coarser particles: at the same time, finer particles become faster and hotter than coarser particles. In other words, finer particles absorb energy from the flame more efficiently and faster than the coarser ones. Figure 1 illustrates the influence of particle size on energy absorption of the powder.

Figure 1. Dependence of the specific particles energy of the sprayed powder from the particle size (same flame with a temperature > 2500°C and a speed > 2000 m s).

Particle size of a powder not only affects its energy, but also the required spray distance. Finer particles need shorter flames and reach their maximum temperatures and speeds after shorter distances. This means that the particle size is decisive for the spray distance. Regardless of the application, burner type and coating material, coating with fine particles requires short spraying distances and coating with coarse particles - long spraying distances. In the HVOF / HVAF sector, the powder size not only determines the necessary spray spacing, but also the burner design, and primarily the nozzle length. The finer the powder, the shorter must be the nozzle. Figures 2 and 3 illustrate the influence of particle size on spray distance and nozzle length for coatings of common tungsten carbide based powders.

Figure 2. Changing the velocities and temperatures of particles in the torches of OD-HVOF burners with a nozzle of 100-140 mm. [Thermico CJS burner with 100 mm nozzle].

Figure 3. Changing the velocities and temperatures of particles in ID-HVOF burners with a very short nozzle. [Thermico ID CoolFlow Mono and ID Red with 13 and 14 mm nozzles].

What is the particle size of a spray powder for practical use? For the above theoretical reasons, the following becomes clear: 1. 2. 3.

For coating with short spray intervals (ID coating), fine-grained powders are absolute necessary, and indeed, the finer the powder, the smaller the inside diameter that can be coated. Fine-grained powders allow coating with low-power burners and short accelerator nozzles. This is especially important for ID applications. For OD applications, fine-grained powders are also interesting because they can greatly reduce energy and gas consumption for coating.

In addition, the layers of small particles are advantageous in the homogeneity of the structure, have no big defects, are smoother, and producible with low continuous layer thicknesses (< 50 microns). The list of advantages of fine-grained powder is so convincing that one must ask why these powders are hardly produced and seldom used in thermal spraying processes. The answer to this question is well known and quite simple: fine-grained powders tend to form lumps, are poorly flowable and therefore hardly suitable for normal powder feeding by the thermal spraying equipment. Explanation for lump formation is the high surface energy of the fine-grained powder (particles are attracted to each other by means of electrostatic forces). The finer the powder, the more it clusters (surface energy increases with decreasing particle sizes proportional to the square of the diameter reduction). For some powders, the average particle size of about 10 microns (D50 = 10 microns) in terms of fluidity may already be critical, in other (dense spherical agglomerates WC-Co or WC-Co-Cr), it is about 5 microns, but no powder with D50 90 MPa.

Figure 6. Spray powder of coarse grain cristobalite (50-100 microns) and fine-grained mixture of metals: chromium (0.5-15 microns), nickel chrome (1-10 microns), silicon (< 2 micron) and molybdenum (2- 5 microns).

Figure 7. Coating produced from the powder of Figure 6, applied with Axial III burner (HV-APS) with a spray distance of 100 mm. Amount of cristobalite residues: 2.2 vol.% or 0.4 wt.%. Maximum size of crisobalite particles in the layer: 20 microns. Porosity of the layer < 0.1%, hardness: 900 HV 0.3 . Brinell test with 16 kg load at the interface between layer and substrate has proven an adhesive strength > 100 MPa.

Although presumed that cristobalite in the powder mixture could adversely affect the coating formed by fine particles, the opposite observe in reality. Deposition efficiency of main, fine-grained powders increases! Converted to finegrained powders, the deposition efficiency for ID-HVOF burners reaches 30-40%, and for plasma burners (Axial III) 70-80%. Of course, the loss of cristobalite reduces the overall deposition efficiency of powder mixture, but because of low cost for cristobalite it is not economically problematic. The differences in deposition efficiency between HVOF and HV-APS explain by the aggregate state of particles: In HVOF coating, all particles of the powder mixture remain solid, but during plasma spraying, the metallic particles melt. Molten metallic particles bring better deposition efficiency and require significantly lower speeds than the solid ones. Cristobalite must be solid in both cases. Even with the hottest parameters of HV-APS, cristobalite grains remain solid (large grains do not reach the melting point of SiO 2 , which is at 1710°C). All the small metallic particles including those of molybdenum, with a melting point of 2623°C, melt. Even melting of fine tungsten carbide (melting point about 3000°C) in a flame with a coarsegrained cristobalite is not a problem (explanation for different particle temperatures in the same flame provided Figure 1).

Other advantages of a ceramic carrier From the above example with Dymet, it is clear that a ceramic carrier can also perform functions other than only improving flowability. There is even a complete list of advantages entailed in the addition of coarse-grained ceramic particles (abrasive particles) to the spraying material: 1.

2.

3.

4. 5. 6. 7.

Abrasive particles (in our case cristobalite particles) activate (sandblast) all coated surfaces directly from the arrival of the metallic and/or cermet particles. There is hardly no time for a renewed oxidation of the surface and the adhesion of layer-forming particles to the substrate or underlying layer is massively improved. In a "normal" coating without cristobalite admixtures, the oxide skins on all surfaces limit the adhesion, as well as the cohesion of the layer. This is especially true for HVOF coatings out of metals and cermets, but also APS coatings suffer from oxide skins between metallic particles. The abrasive insert successfully combats the problem of overspray (defects in the layer due to weakly adhering particles): all particles that do not "stick" to the surface with 100% adhesion are blasted away by abrasive grains. The result is a layer with lower porosity and better cohesion. Overspray problems for coatings without cristobalite are more issue for APS coatings, but even for ID-HVOF applications. This problem has not been solved. Due to the impact of large cristobalite particles, each single layer is additionally compacted. Such a "shot peening" additionally reduces porosity of the coatings and improves their adhesion/cohesion. Abrasive particles smooth the surface of the coating and reduce its roughness. Cristobalite fragments in the layer reduce their effective modulus of elasticity and increase their thermal expansion coefficient. Both are very important for operation with temperature change (lower modulus of elasticity and a thermal expansion coefficient of the layer adapted to coefficient of thermal expansion of the substrate allow heating and cooling of the coated component with less stress in the coat).

Limitations and selection criteria for the use of temporary ceramic With the increasing particle speed, cristobalite works more and more as a blasting medium. At speeds higher than 500 m/s, cristobalite as a blasting medium can completely destroy the growing metallic or cermet layer: coating build-up no longer takes place. This means that only HVOF burners with short acceleration nozzles are suitable for powder mixtures of fine-grained metallic or cermet particles with coarse-grained cristobalite. In all high-performance HVOF burners with nozzle lengths > 100 mm, coarse cristobalite particles are accelerated to critical speeds. Even with the HVOF burners with short ( 250 MPa

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Thermal expansion coefficient at RT: 9.0-9.5x10-6K-1

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Thermal conductivity at RT: 10-20 W/mK

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Density: 7.7 g/cm3

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Open porosity: 0

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Closed porosity: < 0.5%

Chemical properties of the coat: Oxidation resistance in air > 950°C, continuous use at temperatures up to 900°C is possible. After the heat treatment ChroSiMol is absolutely resistant to hot sulfuric acid and phosphoric acid in any concentration. ChroSiMol is stable in virtually all saline solutions and hot solutions of alkalines, weak and medium acids including HF, as well as also in hot hydrochloric acid up to 5% concentration. The layer is not stable in nitric acid, chromic acid and hydrochloric acid with a concentration > 10%. Corrosion resistance to various aggressive media is shown in Table 1 (direct comparison with titanium alloy Ti6-4):

Corrosive Media, Temperature, Duration

ChroSiMol

Ti6-4

10%-H2SO4, 60°C, 48h

no corrosion

corrosive damage approx. 0.05 mm

20%-H2SO4, 60°C, 48h

no corrosion


40%-H2SO4, 60°C, 48h

no corrosion

corrosive damage approx. 4 mm

96%-H2SO4, 60°C, 48h

no corrosion


200-300 g/L CrO3 + 2-2,5 g/L H2SO4, 20°C, 72h

no corrosion

no corrosion

200-300 g/L CrO3 + 2-2,5 g/L H2SO4, 60°C, 72h

corrosion because of pores

no corrosion

30%-H3PO4, 60°C, 48h

no corrosion


5%-HCl, 20°C, 48h

no corrosion

no corrosion

5%-HCl, 40°C, 48h

no corrosion


10%-HCl, 20°C, 48h



20%-HCl, 20°C, 1h



37%-HCl, 20°C, 0,5h



15%-HNO3, 20°C, 24h


no corrosion

15%-HNO3, 60°C, 2h


no corrosion

65%-HNO3, 20°C, 24h


no corrosion

HNO3(65%) + 3HCl(37%), 20°C, 24h

no corrosion


HNO3(65%) + 3HCl(37%), 40°C, 24h



48%-HF, 20°C, 48h

no corrosion

not tested (reaction too fast)

5%-HCl + 10%-HF (1:1), 20°C, 100h

no corrosion


96%-H2SO4 + 48%-HF (1:1), 20°C, 48h

no corrosion


10%-H2SO4 + 10%-HF (1:1), 20°C, 100h

no corrosion


20%-NaOH, 60°C, 48h

no corrosion


3,5%-NaCl, 100°C, air ventilation, 500h

no corrosion

no corrosion

Table. 1. Corrosion resistance of ChroSiMol.

Layer structure:

Figure 8. Microstructure of the layer ChroSiMol before and after the heat treatment in air at 900 ° C (substrate: X45CrSi9.3).

Figure 9. Fragment of the structure before the heat treatment.

Figure10. Same fragment after heat treatment.

Figure 11. Layer structure after 460 h at 900°C (substrate: X45CrSi9.3).

Coat „Cr-Cr 2 O 3 “ [Cr/Cr 2 O 3 (80)NiCr(20)] The cristobalite-containing spray powder has been developed for high-speed atmospheric plasma spraying with Axial III burners.

Coating Cr-Cr 2 O 3 is suitable for all metallic base materials with a thermal expansion coefficient between 10 and 15x10-6K-1 (steels, cast iron, titanium alloys, nickel and cobalt base alloys). However, this layer has the maximum effect in high temperature applications and on nickel base superalloy and titanium aluminide substrates.

Apply the coating: •

The application of "Cr-Cr 2 O 3 " takes place by means of atmospheric plasma spraying with the Axial III burner directly on finely sandblasted substrate surfaces.

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The layer thickness can be varied between 50 and 2000 microns.

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After coating of iron, nickel and cobalt base alloys, subsequent heat treatment in air at 900°C is necessary (a metallurgical bond is formed with the substrate).

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In the case of coating the titanium alloys and titanium aluminides, the sprayed layer is immediately ready for use (diffusion bonding is formed during spraying).

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If necessary, the finished layer can be processed by grinding and polishing.

Mechanical and physical properties of the coat: Cr-Cr 2 O 3 has no pores and microcracks and is metallurgically bound to the substrate. After the heat treatment, the adhesion of the layer to the nickel-based alloys is higher than 250 MPa. •

Hardness at RT: 850-950 HV 0.3 .

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Flexural strength (4 points) at RT: 300 MPa

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E-modulus at RT: 95 GPa

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Adhesion to the substrate: > 250 MPa

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Thermal expansion coefficient at RT: 9.5x10-6K-1

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Thermal conductivity at RT: 3,8 W/mK

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Density: 6.1 g/cm3

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Open porosity: 0

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Chemical properties of the coat: Oxidation resistance in air > 1000°C, continuous use at temperatures up to 900°C is possible (only for nickel-base alloys and titanium aluminides). Corrosion protection against corrosive exhaust fumes and ashes (ashes of heavy oil combustion, fly ash) up to 900°C. Corrosion protection against strong oxidizing acids (e.g., HNO 3 ), alkaline solutions and some salt solutions.

Layer structure:

Figure 12. Microstructure of the layer Cr-Cr 2 O 3 before heat treatment in air at 900°C (substrate: Nimonic 80A).

Figure 13. Microstructure of Cr-Cr 2 O 3 after heat treatment in air at 900°C (substrate: Nimonic 80A).

Figure 14. Proof of a very good adhesion to the substrate.

Coat „Cr 3 C 2 -NiCr-Cr“ [Cr 3 C 2 (50)NiCr(20)Cr(20)Al 2 O 3 (10)]

The cristobalite-containing spray powder was developed for high-speed atmospheric plasma spraying with Axial III burners. Coating Cr 3 C 2 -NiCr-Cr is suitable for all metallic base materials with a thermal expansion coefficient between 11 and 17x10-6K-1 (steels, cast iron, titanium alloys, nickel and cobalt base alloys). The layer resembles the known HVOF layers Cr 3 C 2 -NiCr with the decisive difference that the new HV-APS layer Cr 3 C 2 -NiCr-Cr is gas-tight and protects base materials against corrosion and oxidation up to approx. 800°C.

Apply the coating:

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The application of "Cr 3 C 2 -NiCr-Cr" takes place by means of atmospheric plasma spraying with the Axial III burner directly on finely sandblasted substrate surfaces. The layer thickness can be varied between 50 and 700 microns. After the coating, subsequent heat treatment at 600-800°C is desired, but not absolutely necessary (heat treatment improves adhesion to the substrate and increases the layer hardness). If necessary, the finished layer can be processed by grinding and polishing.

Mechanical and physical properties of the coat: Cr 3 C 2 -NiCr-Cr has no pores and microcracks. Before the heat treatment, the adhesive strength to all suitable base materials is higher than 100 MPa. After the heat treatment, the adhesion of the layer to all substrates is higher than 150 MPa. •

Hardness at RT: 900-1150 HV 0.3 .

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Flexural strength (4 points) at RT: 350 MPa

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E-modulus at RT: 205-215 GPa

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Adhesion to the substrate: 150-200 MPa

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Thermal expansion coefficient at RT: 10.1x10-6K-1

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Thermal conductivity at RT: 9,1 W/mK

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Density: 6.14 g/cm3

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Open porosity: 0

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Chemical properties of the coat: Oxidation resistance in air > 900°C, continuous use at temperatures up to 800°C is possible.

Cr 3 C 2 -NiCr-Cr is virtually completely stable in weak acids, salt solutions and hot alkalis, even in cold sulfuric acid, nitric acid, chromic acid and hydrochloric acid (< 5%).

Good resistance in hot sulfuric acid, but not resistant to hot hydrochloric acid ( > 5%).

Layer structure:

Figure 15. Microstructure of the layer Cr 3 C 2 -NiCr-Cr (substrate: Nimonic 80A).

Figure 16. Comparison with the commercial HVOF layer of Cr 3 C 2 -NiCr (right picture with cracks and pores).

Figure 17. Proof of good adhesion to the substrate.

Summary The new method of coating with, in principle, non-coatable, fine-grained metallic and cermet powders dramatically expands the range of coating materials and possible new coatings. For HVOF technology, the use of fine-grained cermet powder with coarse-grained cristobalite carrier means first and foremost the possibility of producing high-quality internal coatings with small internal diameters. Cermetcristobalite mixed powders are also very interesting for first-generation low-power HVOF burners (short, air-cooled nozzles). The use of new powders on air-cooled HVOF burners such as Diamond Jet, GTV-GLC, Termika 3, HypoJet 2700 and similar can bring massively improve their layer qualities and close to the qualities of modern, powerful but also expensive burners we JP 5000, K2, CJS and others. An application of cristobalite-containing new powder is even more interesting for the HV-APS and, specifically, for Axial III plasma torches: Axial III is the only plasma torch with axial powder feed and, therefore, ideally suited for the processing of powder mixtures with different particle sizes. If with "normal" radial plasma powder feed you need a narrow particle size distribution, so Axial III is a particle size distribution of 1 to 200 microns no problem! This feature opens up a unique opportunity for the development of many new powder blends that are impossible for all other burners except Axial III. Since these unusual powder mixtures can produce very good layers, we have already proven with the layers "ChroSiMol", "Cr-Cr 2 O 3 " and "Cr 3 C 2 -NiCr-Cr".